Chinese exploration teams have drilled their deepest borehole yet in the "roof of the world".

They have punched a seven-kilometre borehole into the Tibetan Plateau in their bid to tap the region's oil and natural gas resources.

It is the deepest borehole ever drilled at such extreme altitudes, according to mainland scientists who are following the project.

Tibet's remoteness, thin air and lack of infrastructure have so far saved it from the unchecked exploration and extraction of fossil fuels and minerals elsewhere in the country.

But the government wants to lessen the country's dependence on oil imports and is funding domestic scientific research to the hilt.

The latest project is shrouded in secrecy. Professor Li Haibing, a researcher with the Chinese Academy of Geological Sciences, would not reveal the project's location and declined to identify which state-owned oil companies were active in the region.

But Li, who led one of the largest scientific drilling projects in Tibet, said: "Tibet's altitude and geology make it among the world's most difficult drilling locations. Fragmented [geological] structures, prone to collapse, increase the risks."

Li, who works for the academy's Institute of Geology, also noted that "oxygen scarcity at higher elevations drains workers' energy considerably".

China has been keeping a low profile about its exploration of resources in Tibet due to the sensitivity of the region, which has seen growing political and religious strife.

The two largest state-owned oil and gas companies, China National Petroleum Corporation (CNPC) and China Petroleum and Chemical Corporation (Sinopec), did not respond to the South China Morning Post's queries on projects in Tibet.

But information on their websites indicate that both firms have had footprints in the region for nearly 20 years. CNPC began exploring the Qiangtang Basin in central Tibet in 1995 and subsequently estimated the basin's oil reserves at 10 billion tonnes, or more than 70 billion barrels.

In 1997, Sinopec established its first exploration centre in Nagqu county, with the aim of mapping the surrounding area with detailed seismic surveys and experimental drilling.

Li said the central government was reviewing a proposal for a new "deep-earth" exploration project "submitted by the nation's most prominent geologists" to drill wells more than 10 kilometres deep to obtain study samples, with Tibet an area of the greatest interest.

Last August, the China Geological Survey, under the Ministry of Land and Resources, signed a 20 million yuan (HK$25.1 million) exploration agreement with Sinopec after the Tibet region showed "enormous oil and natural gas potential", according to the ministry's website.

The discovery of commercially viable flows of oil and natural gas in Tibet has the potential to develop the region's economy. Tibet has one of the lowest gross national products among China's administrative regions.

Professor Wei Wenbo, a geologist with the China University of Geosciences and an expert on Tibetan geological conditions, said scientists continue to debate Tibet's oil and gas potential.

It is hoped that samples from the seven-kilometre borehole will clarify some of the questions about the region's hydrocarbon and mineral resources.

Wei said: "It is one of the last virgin territories for natural resource exploration on the planet, drawing interest from miners and drillers at home and abroad."

The world's deepest borehole - the Kola Superdeep Borehole in northwestern Russia at 12,262 metres - was drilled by the former Soviet Union in the late 1980s.

Wei warned against a rush to exploit Tibet's rich mineral veins.

"Exploration and extraction of minerals and hydrocarbons in Tibet will require massive capital expenditure," he said, noting that infrastructural construction and drilling at such high altitudes drives up labour and logistics costs.

Wei said mining projects had the potential to irreversibly mar Tibet's fragile ecosystems. "Environmental impact studies must be undertaken and risks assessed before commercial projects are approved," he added.